Automatic servo disconnect circuit



signal.

`.atthe input of the amplifier.

Unite 3,102,983 AUTOMATIC SERV() DHSCON Nh CTlCmCUlT `Wheeler `M. Turner, Santa Barbara, Calif., assigner to ,Arnoux Corporation, Los Angeles, Calif.,.a corporation of yCalifornia Filed Mar. 10, 1959, Ser. No. 798,395 8 Claims. (Gl. 328-150) in which reference signals are periodically sent over the link and compared with a lixedreferenca the difference being used to compensate or correct for errors introduced `by the transmission link.

Followup servos ofthis type are capable of correcting over a .substantial range of variations of the reference signals, but it is sometimes desirable under specific operating conditions to limitthe range of inputreference signals overwhich the servo :will corerct. For example, if abnormal or transient conditions are present which take the input reference signals out of the normal range,it is desirable to disconnect `the servo loop to prevent the servo from attempting to compensate fully during the abnormal or transient conditions. Further it is desirable to hold the output of the servo at its existing -level until the abnormal or transient condition is terminated.

The present circuit provides a novel disconnect circuit `for a closed loop servo system which is arranged tov automatically hold the output of the servo at its existing level whenever the input exceeds a predetermined range.

The circuit has been `found to be particularly useful in multiplex data transmission systems of the type employing zero and -fullscale servo regulated reference signals, such as described in the above-mentioned patent, but in addi- `tion has-general application to servo systems and other signal level responsive devicesin which it is desired to t States Patent F Patented Sept. 3, 1963 to the same assignee, `there is `described a decommut-ating system for-time shared pulse-amplitude modulation signals.

` 4T he system therein described comprises, as shown in the limit response to a particular range of input levels and to automatically disconnect the input and maintain the output at kthe existing level in response to transients or abnormal conditions in which the input level exceeds the predetermined range.

In brief, the invention provides a circuit in which an input signal of varying D.C. level is applied to a directcoupled amplifier having a capacitor connectedrbetween the input and output of the amplifier, the input signal being connected to the .amplifier input through a series diode. "The diodefis normally forward biased by current passing through -a gas discharge device. Means is provided where- `by whenever the input exceeds predetermined limits in either direction the `gas discharge device is extinguished andthe diode is back-biased to provide a highimpedance The capacitor, as a result of the back-bias of the diode and extinguished gas tube, has no discharge path. Hence the capacitor `retains its charge, maintaininga constant level in the input with relation tothe output as determined by the existing charge on `the capacitor. y

`For `a better understanding of the invention reference should be had to the accompanying drawing, wherein:

FIG. 'l isa block diagram of one system incorporating the servo `disconnect Afeature vof the present invention; and FIG. 2 is a `schematic Iwiring diagram of the automatic i disconnect circuit.

Ilnlcopending application Serial No. 798,456, `iiled Mar. 110, 1959,` inthe-name-of` the present inventor and assigned l mined frange.

"the D.C. amplifiers 284 and 34 respectively. vtimeithe automatictdisconnect-circuits 26 and 32 are ar- Aranged to maintain the output "of the D.C. ampliiiers `at block diagram of FIG. lof'thegpresent application, a suitablereceiver `and discriminator circuit indicated generally at 10, the' output of which provides the time shared pulse information signal derived from a remote telemetering station (notshown). The time shared pulse informationsignalis `ampliliedin anA.C. -amplier `12. having a gain control circuit 14. The output of the AC. amplier is applied to a clamper circuit ^16ffor restoring the D.C.

level to the time sharedpulse information. `Thebase `level of the` pulse train atithe output of the clamper is set i by alevel control circuit 18.

The output of the clamper 16 is then applied to a pulse Shaper circuit for` deriving noise-free'information pulses whichare then applied to a decommutating or channel separator and demodulator circuit 22 by which the pulse info'rmation'is-divided into the separate channels and demodulated toderive the respective channel analog signals `in-*decommutated form.

Automatic control of thegain of the amplifier 12 and ftheibase level produced by the clamper `16 may` be servo "controlledfrom 'Zero land yfullscale reference information transmitted in the rst `and second channels yfrom the sending station. For instance, pulses whosel .amplitudes representlzero input signals for that channel at the sending station provide a 4zero reference at the output of the #d channel from the-channel separator and `demodulator circuit-22.` Similarly-pulses whose Iamplitudes represent full- `scale provide a fullscale reference at the output of the #12 channel from the channel separator and decommutator wcircuit-"22. i

For servo control, the #l channel output is connected `to acomparison circuit ywhere the output level is compared `with aitixed reference voltagej The diiferenceis applied `through -an automatic servo disconnect circuit 26, ampliiiedby a`D.C. ampliiierlZS, andriappliedto the level control circuitlSto adjust the level ofthe clamper `circuit `16 lso as to produce a` corrected zero output level from the #1 Channel.

A lSimilarly the #Z channel is applied to a` comparison circuit '30 where the liullscale output is compared with a lixed reference voltage, the difference being applied through 2in-automatic disconnect circuit V32 and 1D.C. amplier 34 to thefgain control circuitllll. -Thusthe gain of the amplilier I2 is automatically controlled to provide a corrected Vfullscale level at the output of the #2 channel. Once the p `gain of the amplifier `-I2 and level of the clamper circuit 16 `are set for fullscale and .Zero reference respectively,

proper gain and level isestablished for all of the informa-` tion channels.

The automatic disconnect circuits26 and '32 are arranged as hereinafter described in detail to automatically disconnect the output from the comparison circuits `from l At the same their existing levels indefinitely or until the output of the comparato`r..fcircuits` returnfto `values Within a predeter- -Referringto FIG. 2, `a schematic circuit diagram is shown for the automatic disconnect circuit `26 of FIG. `l.

`Asshown inFIG. 2, the comparison circuit comprises av resistance voltage dividernetworkicomlected between a +250 v. and 250 v. fixed voltage reference. The comparison cireuitincludes two lxed resistors an-d`38 on eitherside of Va potentiometer 40. The output from the comparison circuit is derived fromthe sliding contact of the `potentiometer 40. The input, derived from the #l channel of the channel separator `and demodulator "-22, is `connected Ito Vthe junction between ltheresistor J33` voltage sources.

andthe potentiometer 4d. y erably set so that theoutput of the comparison circuit is l three or four volts positive with relationship to the level of the #l channel input,v which input normally ranges aroundzrero volts potential with respect yto ground. v.

The output derived from the sliding contact of the v potentiometer 40 is connected to the DC. ampliier'28 v'through a iirst series resistor 42,'a diode 44, and a sec- The potentiometer vis pref-i 4i drives the plate of the second stage positive andpextinguishes the neon tube 48. 1

At the same time the diode 44, which is normally fori ward biased, is back-biased when the neon tube 48 is ond series resistor '46. The diode 44 is normally forward biased by '.'means of a neon tube or other gas discharge device 48 which is connected through a large series resistorStl to a normally negativepotential derived on a l.voltage divider consisting of a resistor 52, la neon or Votherf'gas A discharge device 54, and resistors 56 and 58 connected'between vthe positive and negative reference 50, ve'ry small current is drawn through the gas discharge tube 48.

From .the circuit thus'fardescriibed it Iwill be apparent that any changes in the level at the#1 channel input to the comparison circuit will change the Ylevel at the'inputl of the amplifier 28 to produce acorresponding amplified change in level at the-output of the amplier 28.

Because of the relatively large resistorl During automatic disconnect operation, the neon `tube 48 isextingu-ished and the diode 44 -is back biased so as to provide a very high input impedance and, in etect, disconnecting theinput to the amplifier 28. A capacitor extinguished by virtue of a pair of diodes 86 and 88 connected respectively between the anodeof the diode 44 and ground and the anode'of the diode 44.1and the output of the first stage at the junction of the resistors 74 and 76. Undernormal operating conditions the anode of the diode 44 is `positive and both the diodes 86 and 88 are back bia'sed'so as to be nonconductive. It a negative transient is applied to the input of the automatic disconnect circuit of sufficient magnitude to extinguish the tube 48, the diode 86 is rendered conductive through the resistor 42. The diode 86 clamps the anode of the diode 44 to a potential slightly negative with respect to ground, and since the input to the amplifier 28 is normally slightly positive with" respect to ground, the anode 44 is back biased, thereby providing a high impedance to .the capacitor 6l?. The diode 88 remains back biased since its cathode is driven more positive by the negative-going transient on the input of the iirst stage 64.l

When a positive-going transient or step function .voltageV is applied to the input of the automatic disconnect circuit which is of suiiicient magnitude toy extinguish the tube i 48, since the cathode of the diodel normally `goes nega- `60 connects the output of the amplifier 28 to the input v andnormally assumes a charge determinedby the diff- Iferencevin level of the input and output of the ampliiier v28.` Duri-ng disconnec-t operation, with the neon tube 48 extinguished and the diode'44 back-biased in a manner v hereafter described, no discharge path for the capacitor 60 is `available and it retains itsv charge indefinitely thereby maintaining the level at the output of theampler 28 at itsexisting level at the time vthewinput is automatically disconnected. i v f ltivewith respect to ground potential, the diode 88 is also rendered conductive. This causesthe diode 86 to conduct, clamping the anode of the diode 44 at a potential slightly negative with'respect to ground and backabiasing the diode 44.

To cut 0E the vgas discharge tube 48,- the automatic discharge circuit sensesthe level 'at the output of the comparison circuit at the sliding contact of the potentiometer 40j through an on-oi switch 62. Only when the A switch 62 isfclosed is the automatic disconnect -feature in operation; j The disconnect circuit includes two directcoupled stages including triodes 64 and 66. 'I'he first stageincludes'acathode resistor 6,8 and plate loa'dresistor 70. `The plate, of the rst stage is connectedto the grid ofthe second stage through a neon tube 72 which provides suicient voltage drop toestablish the proper-bias level of the grid ofthe .second stage. In` series with the neontube 72 is a resistor 74 which is part of a voltage divider network including resistors 76 and 78 returning to the negative potential source. The grid of the second stage is connected through a very large resistor 79 to. the series i junction point between the resistors 74 and 76. l Y Large negative feedback is applied to the grid ofthe iirst stageby connectingthe series junctionpoint between the resistors 76jand` 78 through a resistor 80 to'the grid of the triodey 64. This negativev feedback provides sub-v` stantially unity gain tor the iirst stage. f Thus the irst r stage serves primarilyas a phase reversing means, since the output level moves inthe opposite direction to changes in the level of the input.

Theresistor 52 provides .the plate load resistor for the v Asecond stage 66 which also includes a cathodebias resistor `82.1 If a positive-going transient the form of a step v voltage, -for example, is derived from the #1 channel i11- put, it Will be seen that the plate of the second stage is V,driven positive. This causes the series junction point between the resistors S6 and 58 to become less negative, causing the neon tu-be 48 to be extinguished. v If a negative transient exists at the input to the comparator circuit 24, the lgrid of'thesecond stage is again driven negative vby virue of adiode 84 which bypasses the rst stage. Thus ya negative transientpfor -av s-tep Ifunction voltage also 'Dhus it will be `seen that for both negative or positive going transients exceeding a limited voltage range, the neon tube 48 is extinguished and the diode 44 is back biased thus isolating'the input to the amplifier 28. Whenever the input returns to the normal range of operation, the neon tube 48 is again ignited and forward-biasing of the diode 44 is' restored, permitting the output of the amplifier 28 to again track changes in therefcrence level of the #1 channel input.

The automatic disconnect circuit 32 for the fullscale .servo of FIG. l is identical to -that described in FIG. 2. However, the lcomparison circuitpis arranged slightly .dif-

ferent because the normal level of the #2 channel signal is in a range substantially positive with respect to ground.

Y votherwise operation is identical as described above in connection with FIG. 2.

From the above description it will be recognized that an Aautomatic disconnect circuit has been provided which is particularly` useful in providing `automatic disconnect f of servos When input conditions exceed normal operating limits. The circuit hasthe feature that it is automatically resetting when the transientA conditionk passes and input conditions returned to normal operatingranges.y The circuit is falso unique in that it provides for sustainingvthe n output at the existing level indenitely at the time a transient 'condition occurs. It will be noted that only abrupt changes in the zero or tullscale 4reference signals will operate the :disconnect circuit, Slow changes vare Washed out by the action of the servo loopt VdThe input to the disconnect circuit can only change in response to f transients which change the reference level Vrtoo quickly for the servo to follow. What is yclaimed is:

1. In `a closed loop followup servo systerrn apparatus amplifier changing with changes in the voltage level of the control signal, means responsive to the control signal -for extinguishing the discharge tube when the control signal varies in voltage above and below predetermined limits, a iirst diode clamping circuit coupled to the series diode for limiting the negative swing yof the control signal input to the amplifier, a second diode clamping circuit coupled to the series diode for limiting the positive swing `ol the control signal input including means tor reducing the clampinglevel as the control signal input increases beyond a predetermined level, whereby the series diode is always back-biased when the gas discharge tube is eX- tinguished. 1

2. Automatic disconnect apparatus for automatically disconnecting an input terminal from an output terminal when the potential applied to the input terminal exceeds a normal operating range, the apparatus comprising means including a first diode providing a current path between the input terminal and the output terminal, means for normally biasing the series diode ina conductive state including a normally conductive gas discharge tube connected to the output terminal to provide a current conductive path :for the rst diode, means responsive to the potential level of the input terminal for extinguishing the discharge tube when the level of the input terminal varies in voltage `above and below said normal operating range, a first diode clamping circuit connected to the series diode for limiting the negative swing cf the series diode in relation to the input terminal, a second diode clamping circuit coupled to the series diode for limiting the positive swing of the series diode in relation to the input Iterminal including means for reducing the clamping level as the input terminal increases beyond a predetermined level, whereby the series diode is always back-biased when the gas discharge tube is extinguished.

3. An automatic signal disconnect circuit comprising a signal source of varying potential with relation-to ground reference, the potential normally varying within a limited range, a resistor and iirst diode connected in series with the output 'of the signal source, a high resistance current path including a gas discharge tube connected to the lirst diode, means tor normally applying potential across the resistor, iirst diode and `gas discharge tube of sufficient magnitude to maintain the igas discharge tube conductive, the tirst diode being connected to be tor-ward biased by said applied potential, means responsive to changes in potential of the signal source above or below the normal `operating range for reducing the potential across the ygas discharge tube and extinguishing the tube, a tirst clamping diode connected to the series junction point between the resistor and iirst diode and connected to Ia fixed potential reference, the iirst clamping diode normally being back-biased by the potential reference to render it nonconductive, a second clamping diode, phaseinverting means connected tothe signal source to provide a potential source that ydecreases when the potential of the signal source increases, the second clamping diode being connected to a series junction point between the resistor and said first diode 'and connected to the output of the phase-inverting means, the second clamping diode normally being back-biased by the loutput of the phaseinverting means when the signal source is the normal operating range, the output from the automatic disconnect circuit being derived trom the series diode.

4. Apparatus as dened in claim 3 further Iincluding a direct coupled amplifier having the input :thereto being connected to the first diode through a series resistor, and a capacitor connecting the youtput of the amplifier to the input, the capacitor iixing the level at the input to the amplifier when the -gas discharge tube is extinguished and the iirst diode is back-biased.

y5. A switching circuit having an input terminal and an output terminal, the switching circuit automatically disconnecting the output terminal from the input terminal when the potential applied to the input terminal exceeds either the upper limit or lower limit of a predetermined operating range, the circuit comprising means including an isolation diode connecting the input terminal to the output terminal, high impedance means connected in series with the diode for normally lbiasing the diode in the forward direction with relation to the input terminal, whereby potential changes at the input terminal produce potenti-al changes at the output terminal, means responsive to` changes in the potential tof the input terminal which exceed either the upper limit or the lower limit of the operating ran-ge Ifor electrically disconnecting said high impedance means front the diode, and means responsive to changes in the potential of the input terminal which exceed either the upper limit or the lower limit `of the operating range for back-biasing the diode when the high impedance means has been disconnected to provide `a high impedance :at the output terminal, said lastnamed means including means Afor clamping the diode to a iined reference potential.

6. Apparatus as defined in claim 5 wherein the clamping means includes irst and second diodes connected in series with each other, the series junction point of the two series diodes being connected to the isolation diode on the opposite side `ot the isolation diode from the output terminal, and phase inverting means coupled between the input terminal land the two series ldiodes -for varying the potential across the series diodes inversely with changes in the potential of the input terminal.

7. Apparatus as defined in claim 5 wherein said high impedance means includes a gas discharge tube and a high series lresistance connected to the output terminal side of an isolation diode, and means responsive to changes in the potential at the input terminal in excess of the limits 4of said operating range for decreasing the potential across the gas ldischarge tube and series resistor to a value less than is required to sustain the ionization of the discharge Itube.

`8. Apparatus `as defined in claim 6 wherein said lu'gh impedance means includes a gas discharge tube and a high series resistance connected to the output terminal side of an isolation diode, and means responsive to changes in the potential at the input terminal in excess of the limits of said operating range tor decreasing the poten- :tial across the ygas discharge tube and series resistor to a valve less than is required to sustain the ionization of the discharge tube.

References Cited in the file of this patent UNITED STATES PATENTS 1,984,065 Greenwood Dec. lil, 1934 2,247,468 Barr et al I-uly 1, 1941 2,652,488 Smeltzer et lal Sept. 15, 1953 2,829,247 Thomas Apr. l, 1958 2,930,987 Groce Mar. 24, 1960 

1. IN A CLOSED LOOP FOLLOWUP SERVO SYSTEM, APPARATUS FOR AUTOMATICALLY DISCONNECTING THE SERVO LOOP AND LOCKING THE SERVO OUTPUT TO THE EXISTING OUTPUT LEVEL, THE APPARATUS COMPRISING A DIRECT CURRENT AMPLIFIER, A CAPACITOR COUPLING THE OUTPUT TO THE INPUT OF THE AMPLIFIER, A FIRST DIODE IN SERIES WITH THE INPUT OF THE AMPLIFIER FOR COUPLING A CONTROL SIGNAL TO THE AMPLIFIER INPUT, MEANS FOR NORMALLY BIASING THE SERIES DIODE IN A CONDUCTIVE STATE INCLUDING A NORMALLY CONDUCTIVE GAS DISCHARGE TUBE CONNECTED TO THE INPUT OF THE AMPLIFIER TO PROVIDE A CURRENT CONDUCTIVE PATH FOR THE FIRST DIODE, THE INPUT LEVEL OF THE AMPLIFIER CHANGING WITH CHANGES IN THE VOLTAGE LEVEL OF THE CONTROL SIGNAL, MEANS RESPONSIVE TO THE CONTROL SIGNAL FOR EXTINGUISHING THE DISCHARGE TUBE WHEN THE CONTROL SIGNAL VARIES IN VOLTAGE ABOVE AND BELOW PREDETERMINED LIMITS, A FIRST DIODE CLAMPING CIRCUIT COUPLED TO THE SERIES DIODE FOR LIMITING THE NEGATIVE SWING OF THE CONTROL SIGNAL INPUT TO THE AMPLIFIER, A SECOND DIODE CLAMPING CIRCUIT 